1. Field of the Invention
The present invention relates generally to coated plastic substrates such as polymer-based optical components including window panes and mirrors, and specifically to lightweight and durable synthetic resin optical components resistant to abrasion and war-page and methods for the manufacture thereof.
2. Description of Related Art
Coated plastic substrates and optical components are commonly used in various applications. The use of lightweight plastic substrates are frequently desirable but have presented problems for long-term use.
Mirrors typically have a multilaminate configuration. In particular, mirrors are typically formed by selectively depositing a series of compounds on a glass substrate material. These layers generally include a reflective layer and a protective back-coat layer covering-the reflective layer. The reflective layer is commonly formed from a thin film of aluminum, chromium, rhodium, or silver. In industrial applications, aluminum is typically used in place of silver due to its high reflectivity and low cost. The protective back-coat layer serves a multiplicity of functions, such as protecting the reflective layer from humidity. This function is required as the reflective layer, especially if it is formed from aluminum, is easily corroded by moisture. Since the substrate material upon which the reflective layer is deposited is often permeable to moisture, it is important that the protective back-coat layer be substantially impermeable to moisture in order to provide an effective encasement for the reflective layer. The back-coat layer also serves as a mechanical barrier to, for example, impact damage from airborne particulate matter. A properly configured and applied back-coat layer thus assists to provide a durable mirror.
Due to the high production costs related to glass mirrors, significant research has been undertaken to develop a durable, low-cost plastic mirror employing a synthetic resin substrate material. Furthermore, due to the shatter-proof nature of synthetic resin mirrors, their use is preferred in automobiles over conventional glass mirrors in order to improve the safety of the automobile. The primary focus of this research has been in connection with dynamically stable and substantially optically clear thermoplastic or thermoset resins, such as polymethyl methacrylate (PMMA). As a result of these efforts, a method for sequentially depositing an aluminum reflective material and an impermeable back-coat layer on a resin substrate material has been developed.
The main problem associated with synthetic resin mirrors is their significantly limited operational service life resulting from warpage or distortion of the mirrors due to the hygroscopic properties of thermoplastics or thermoset resins. Unlike their glass counterparts, mirrors formed with a thermoplastic or a thermoset resin as their substrate material gradually absorb moisture from the surrounding atmosphere. Over time, the moisture so absorbed corrodes the reflective layer. Further, the absorption of moisture, coupled with variations in other climatic conditions, causes the thermoplastic or thermoset resin to expand and contract. Compounding these problems is the fact that the back-coat layer is, typically, not affected by humidity or other climatic conditions. The back-coat layer thus acts to prevent the smooth linear expansion and contraction of the thermoplastic or thermoset resin substrate. Furthermore, the moisture permeability of the various coatings applied to both sides of the synthetic resin substrate often lead to different amounts of moisture being absorbed by the opposing surfaces of the synthetic resin substrate, thus resulting in uneven expansion and contraction on both sides of the substrate. These conditions all interact to produce distortion to the image produced by the reflective layer of the plastic mirror and a related loss of optical clarity. As the mirror ages, this degradation only becomes more acute.
In order to reduce the susceptibility of synthetic resins to hygroscopic effects, it has been proposed that a hardening material be applied to the thermoplastic or thermoset resin substrate before deposition of the reflective layer. Currently organosilicon polymers are the preferred hardening material. These polymers are preferred due to their ability to provide protection against impact damage and their high optical clarity when fully cured. Although organosilicon polymers are the best available material for this purpose, these polymers are not totally impermeable to water. Thus, although partially effective, these polymers do not provide a complete remedy to all of those issues related to the use of a thermoplastic or thermoset resin substrate material in connection with a mirror apparatus.
A need exists for a mirror apparatus and other optical components that do not suffer from the foregoing disadvantages and limitations. In particular, a need exists for a mirror apparatus formed using a thermoplastic or thermoset resin substrate that will remain substantially unaffected by ambient environmental conditions.